Implement z_getnewaccount and z_getaddressforaccount or similar RPC method

[oxarbitrage]

We plan to eventually integrate zallet into Zebra's qa tests (related ticket). Most of the wallet tests we intend to port depend on the getnewaddress RPC method, but that is deprecated and can easily be replaced by z_getnewaccount and z_getaddressforaccount. [Edited by @daira per the Zcashd Deprecation meeting on 2025-02-27.]

The goal here is to implement this functionality in zallet.

• This may be blocked by Key material storage #18 and potentially related issues.
• It might also be addressed as part of Implement zcashd wallet methods that can already be satisfied by zcash_client_backend #20, but prioritizing it separately is beneficial since generating a new account/address is a fundamental wallet function.